The present invention relates to windmill control, and it is concerned particularly with control of the pitch angle of the windmill's turbine blades during operation.
The power that a windmill-driven generator produces increases with wind speed, and so does the wind force experienced by the windmill. High wind speeds can cause the generator to exceed its rated power level and thus damage the generator. Other windmill parts are also subject to damage or excessive fatigue when the wind force is too high. This is true not only of windmills used for power generation but also of windmills used, for example, to pump water. So there must be some way to control the wind force experienced by the windmill. One way is to control blade pitch.
In the case of a windmill driving an induction generator, for instance, the speed of windmill rotation must be least great enough to provide some "slip" between the generator rotation and the phase advance of the voltage on the power grid. As the speed increases, so does the power generation, but excessive speed can overheat the generator or mechanically strain the windmill, causing damage or premature wear. Too little speed, on the other hand, results in inefficient use of the system. Therefore, it is important to control the speed of the turbine by adjusting the pitch of its blades.
Although it is desirable to keep the turbine speed exactly at the optimum power-generation level, to do so would require very frequent pitch changes and result in excessive pitch-control-mechanism wear. It is accordingly an object of the present invention to control windmill output power accurately while avoiding excessive wear.